It totally depends on the flowers. Something like tulips will start looking tired quite quickly for example – best always to ask the florist what are the best options for longevity. If the flowers you want are prone to wilting, consider silk ones instead – these days they are very realistic. My florist sprayed something on my bouquet which kept it looking great for the whole day.

@newname_99: Calla lilies should be fine, just make sure you keep them in a vase right up until your ceremony. @Cariad is right, you’re going to want to make sure you get flowers that will stand up well in the heat (I wouldn’t recommend gerbera daisies, tulips, or hydrangeas).